In many applications, such as in hybrid motor vehicles, there is a need for two high torque electric motors. Depending on the particular application, the two motors may not need to produce the same torque, but both can be characterized as high torque. The torque produced by an alternating current electric motor is directly proportional to, among other factors, the current flowing through the motor windings and the square of the diameter of the motor, usually defined as the diameter of the airspace between the rotor and stator of the motor. Hence, two ways to increase the torque of a motor are either to increase the diameter of the motor or to run the motor at a higher current. Again, depending on the application, it may not be possible to arbitrarily increase either or both the current through the motor windings and the motor diameter. Instead, the current through the motor windings may be limited by the capability of cooling the motor, and the diameter of the motor may be limited by the space available in the motor vehicle or other application.
In prior applications the need for two high torque electric motors has been met by two separate motors. The two motors may be mounted side by side along a common axis, or the two motors may be mounted on different axes. In order to develop the required amount of torque from each of the two motors, they have been built with large diameters and/or have been designed for large winding currents. In many applications, however, these approaches to providing the necessary torque are limited by the physical constraints of the application. Expanding the diameter of the motors may not be possible given certain space restrictions. The space allocated for the electric motors may be limited, for example, in a motor vehicle or the like. Increasing the winding current for a motor is not as efficient for increasing torque as is increasing the motor diameter because winding current has only a first power effect on torque in contrast to the second power effect of motor diameter. In addition, increasing the winding current leads to increases in Ohmic heating within the motor since Ohmic heating is proportional to the square of the current. The increase in heating, in turn, requires more cooling which increases cost and often weight and/or space.
Current two-motor designs can only deliver high torque at the cost of either increased volume and/or increased winding current with the attendant increases in cooling systems necessary to maintain an acceptable motor temperature. Accordingly, a need exists for an improved dual electric motor that is capable of providing the torque required for an application without unduly increasing motor volume or motor winding current.